1. Field of the Invention
The present invention relates to an apparatus and method for welding strap connections between inner grid straps of a spacer grid using a laser tool, and a spacer grid manufactured using the same and, more particularly, to an apparatus and method for welding strap connections between inner grid straps of a spacer grid using a laser tool, and a spacer grid manufactured using the same, in which the strap connections between the inner grid straps of the spacer grid are seam-welded continuously or intermittently using the laser tool, so that the structural strength of the spacer grid is increased, a spattering phenomenon, in which a welded material is spattered during welding, is reduced, and thermal deformation is reduced, which makes fine welding possible, so that the welding bead of each strap connection between the inner grid straps can have a small size, and the number of welding defects can be reduced, and in which the flow resistance of a coolant is reduced, so that the pressure drop of the coolant can be reduced, and the load on a flow pump for the coolant can be reduced.
2. Description of the Related Art
In general, a spacer grid functions to support and hold a bundle of nuclear fuel rods by arranging grid straps, each of which has a spacer grid spring and dimples, in a matrix array and welding their intersections.
Currently, a commercially available spacer grid is used by spot-welding the intersections between the grid straps arranged in a matrix array. Therefore, the welding method and the quality of the intersections have a great influence on the structural performance of the spacer grid.
Such a spacer grid can increase the structural performance by improving the welding method and quality of the intersections, and thereby the characteristics of the nuclear fuel can be greatly improved.
The spacer grid for fuel rods in a light-water reactor basically has a weld structure at the intersections between inner grid straps, intersections between inner and outer grid straps, and corner joints between outer grid straps. Currently, when the spacer grid is manufactured by a domestic fuel producer, the corner joints between the outer grid straps are spot-welded using a laser, and the intersections between inner grid straps are also spot-welded using a laser.
Meanwhile, a current process of spot-welding the intersections between inner grid straps is performed through laser welding using six or seven spots. As such, a spattering phenomenon, in which a welded material is spattered around the spacer grid during welding, occurs as a result of the welding.
In addition, in the spot-welding process, high heat generated from the laser can cause a change in the dimensions of the spacer grid and thermal deformation. Hence, additional care must be taken.
As described above, the conventional method of welding the intersections between the inner grid straps of the spacer grid in order to support the nuclear fuel rods, as illustrated in FIG. 1, is performed by welding strap connections 150 between the inner grid straps 140a and 140b of the spacer grid 100. In this welding method, a welding bead after welding has a large size, fine welding is impossible, and the pressure drop of the coolant flowing in the reactor core becomes high due to the large welding bead.
The conventional method of welding the spacer grid gives rise to the spattering phenomenon and thermal deformation after spot-welding. Thus, fine welding is impossible, the size of the welding bead of the intersection in the spacer grid is large, and the flow resistance of the coolant increases. Ultimately, the pressure drop of the coolant is increased, which is responsible for increasing the load of a coolant flow pump. In order to increase the structural strength of the spacer grid and decrease the pressure drop of the coolant, a new method of welding the spacer grid must be developed.
To this end, it is critical to develop a welding technique using a super-precision laser and an optical transmission mechanism. In other words, spacer grid welding using the super-precision laser entails little thermal deformation, and makes fine welding possible, so that the welding bead can have a small size, the pressure drop of the coolant can be reduced, and the structural strength of the spacer grid can be greatly increased.
Accordingly, the welding method to be developed in the future is technology essential for the development of nuclear fuel for a nuclear power plant, and it is expected that such technical development will be actively carried out in advanced countries sooner or later.
In particular, the intersection spot-welding method performed at the upper and lower end of the spacer grid has a large welding bead and entails a large pressure drop, and reduces the flow area, and thus it greatly increases the load of the coolant pump, and decreases the design freedom of a nuclear reactor structure. For this reason, in order to increase the mechanical/structural stability of the spacer grid for the nuclear fuel rod bundle and decrease the pressure drop of the coolant, the introduction of new high-tech welding technology is required.